Using several Calorie calculators, I guess I need around 2700 cal a day to maintain weight.

If I burn around 700 calories a day through exercise, I would then assume if I eat 2700 + 700 calories a day then I should maintain my weight.

I am trying to lose about 5kg for peak fitness, and understand I need to reduce intake per week by around 7700 cal to lose ONE kg.

So, if I drop my intake to 2000 cal a day, and ensure I average around 700 a day in exercise, then I am essentially 2700 + 700 - 2000 = 1400 cal a day in deficit. Looking at the week, I am 9800 cal in deficit, which should be around ONE kg. Correct? Basic math gives the intake I should stick to.

If the above is right, ny question is this. Thur morning, I weighed myself, 80.9kg. During the day I did NO EXERCISE and ate around 1300 calories before dinner. Went to Hog's Breath and had steak and chips and coke - could be NO MORE than around 1500 cal for dinner. Total for the day around 2800. This morning, same conditions (first thing, empty bladder, same clothes), I weighed 82kg. If it takes 7700 cal to burn off 1kg weight, how can I put on OVER 1kg in weight when I have eaten just over my normal daily intake?

Sorry about the long post, just trying to understand how that can happen.

A person's weight will vary up to a kilo or two day to day even if the amount of fat they are carrying remains the same. The variations are due to food bulk, water retention and weighing scale mechanics.

Your momentary weight gain has little to do with fat. True weight-loss due to fat-loss is best measured over weeks and months. A steady programme of Calorie restriction, good quality nutritious food and exercise will result in a steady loss of fat over time, although it will result in daily ups and downs of up to a kilo or more. Don't worry, it all works out fine over time.